Plant-based and dynamic thresholds for mirids in early stage cotton.

There have been a num­ber of reports of mirid infes­ta­tion in cot­ton on the Cen­tral High­lands and at St George. Recent research has focused on the devel­op­ment of plant-based thresh­olds that should make it eas­i­er to make deci­sions about mirid con­trol.

Typical mirid damage to a cotton seedling. Wilting and browning of terminal.

Typ­i­cal mirid dam­age to a cot­ton seedling. Wilt­ing and brown­ing of ter­mi­nal.

How do mirids dam­age ear­ly cot­ton?
In the ear­ly stages of cot­ton, mirids feed on grow­ing tips and leaves caus­ing them to turn black (Fig­ure 1).

As the crop devel­ops, mirids move to the squares (a pre­ferred food source), where they feed on the ovules and anthers (Fig­ure 2). Dam­age to the ovules reduces the pro­duc­tion of aux­ins, the chem­i­cal respon­si­ble for retain­ing squares on the plants. The decline in aux­ins, and the pro­duc­tion of eth­yl­ene in response to feed­ing results in squares shed­ding.

Plant-based thresh­olds to improve deci­sion mak­ing.

The plant based thresh­old is an alter­na­tive to the cur­rent mirid numer­i­cal dynam­ic thresh­olds. Depend­ing on your cir­cum­stances use either the plant based thresh­old or the dynam­ic thresh­old.

Mirid dam­age in ear­ly cot­ton (up to 1st boll set stage) can be com­pen­sat­ed for, pro­vid­ing there are no oth­er stress­es (e.g. mois­ture) or dam­age from oth­er pests. Cot­ton indus­try thresh­olds are cur­rent­ly based on mirid num­bers at dif­fer­ent crop stages (see Cot­ton Pest Man­age­ment Guide), how­ev­er stud­ies on mirid dam­age over a num­ber of years have shown that the rela­tion­ship between mirid num­bers and their dam­age is not always lin­ear. Plant-based thresh­olds are based on per cent squares/bolls reten­tion in con­junc­tion with rela­tion­ship between mirid num­ber and dam­age (per­cent squares/boll loss).

 

Figure 2. Anthers damaged by mirid feeding (left) and undamaged anthers (right).

Fig­ure 2. Anthers dam­aged by mirid feed­ing (left) and undam­aged anthers (right).

Using a plant-based thresh­old for mirids

1. Check fruit reten­tion.
Exam­ine 1st and 2nd posi­tion squares and bolls up to 14/15 node stage of whole plants and there­after assess the top 5 nodes. Cal­cu­late the % reten­tion (see Cot­ton Pest Man­age­ment Guide).
2. Sam­ple mirid pop­u­la­tions
Sam­ple using a beat­sheet to cal­cu­late mirid num­bers per metre (see Giv­ing Mirids a Good Beat­ing. Deutsch­er et al. 2003. The Aus­tralian Cot­ton­grow­er 24(3): 24–27).

The rela­tion­ship between fruit loss and yield is sig­nif­i­cant for high yield­ing crops (≥10 bale/ha), and a reten­tion lev­el of 80% (no more than 20% fruit loss) needs to be main­tained up to 14/15 node stage (Fig­ure 3).

 

Figure 3. Increasing fruit loss results in reduced yield.

Fig­ure 3. Increas­ing fruit loss results in reduced yield.

Stud­ies also showed that 2% fruit loss per week will occur for every mirid above the pop­u­la­tion from pre­vi­ous assess­ment (Fig­ure 4).

Figure 4. As mirid numbers increase, so does the potential yield loss.

Fig­ure 4. As mirid num­bers increase, so does the poten­tial yield loss.

For exam­ple, in your pre­vi­ous assess­ment if you have 82% reten­tion and any num­ber of mirids per metre, one addi­tion­al mirid will result in 80% reten­tion after a week which means you have reached the plant based thresh­old. On the oth­er hand, in your pre­vi­ous assess­ment if you have 90% reten­tion and any num­ber of mirids one addi­tion­al mirid will result in 88% reten­tion after a week; which is well above thresh­old.

Con­trol­ling mirids with insec­ti­cides

Select­ing the right chem­i­cal for mirid con­trol in ear­ly cot­ton is crit­i­cal. Dis­rup­tive chem­i­cals at this stage will cre­ate prob­lems lat­er through the flar­ing of oth­er pests such as mealy­bug, white­fly and aphids.

In the case of mealy­bug in the Cen­tral High­lands, ben­e­fi­cials are cur­rent­ly the only effec­tive means of sup­press­ing pop­u­la­tions. Killing ben­e­fi­cials such as lady bee­tles and lacewings with insec­ti­cides can result in mealy­bug num­bers build­ing up to dam­ag­ing lev­els.